1. Filed of the invention
The present invention relates to a catalyst warming up device of an internal combustion engine.
2. Description of the Related Art
An internal combustion engine has been proposed, in which exhaust gas is purified in the following manner: In order to purify unburnt HC and CO gas as early as possible after the start of an internal combustion engine, an electrically heated type catalyst, which is disposed in an exhaust gas passage of the engine, is electrically energized with current right after the engine has been started. When the temperature of the catalyst has been raised to an activation temperature at which unburnt HC and CO gas is actively oxidized, secondary air is supplied into the exhaust gas passage in the upstream of the engine. Unburnt HC and CO gas is purified by this secondary air.
However, rich mixture is supplied into cylinders of the engine for some time after the start of the engine. Accordingly, even when the catalyst is heated to a temperature not less than the activation temperature of the catalyst, the unburnt HC and CO gas cannot be purified unless a sufficient amount of secondary air is supplied. In the following description, an air-fuel ratio is defined as a ratio of an amount of overall air supplied to a suction air passage of the engine and an exhaust gas passage in the upstream of the catalyst, to an amount of overall fuel. In the case where the air-fuel ratio is lean or the same as a theoretical air-fuel ratio when the catalyst is heated to a temperature not less than the activation temperature and a sufficient amount of secondary air is supplied, a sufficient amount of oxygen exists in the exhaust gas so as to burn unburnt HC and CO gas. Accordingly, the unburnt HC and CO gas is completely purified. However, in the case where the air-fuel ratio of the exhaust gas is rich, time, the sufficient amount of oxygen to burn unburnt HC and CO gas does not exist in the exhaust gas. Accordingly, the unburnt HC and CO gas can not be completely burnt.
In other words, as long as the air-fuel ratio of exhaust gas is rich, the unburnt HC and CO gas cannot be purified even when the catalyst is heated to the activation temperature. Consequently, even when the catalyst is electrically heated, electric power is wasted and no effect can be provided. Accordingly, when the catalyst is electrically heated after the start of the engine and secondary air is supplied to the exhaust passage in the above-mentioned internal combustion engine, electric power is wasted for heating the catalyst if the air-fuel ratio of exhaust gas is rich.